1. Field of Invention
The present invention relates in general to electrical submersible pump (ESP) systems and, in particular, to an improved system, method, and program product for controlling motor terminal voltage under varying load conditions by compensating for cable loss.
2. Background
Electrical submersible pumping (ESP) systems are often used in hydrocarbon producing wells for pumping fluids from within the wellbore to the surface. Within this specification, the term wellbore includes oil wells, gas wells, geothermal wells, carbon sequestration wells, and others as understood by those skilled in the art. A typical ESP system includes a centrifugal pump which is driven by a three-phase AC motor, both located in the wellbore, and a variable speed drive which delivers three-phase power to the motor, located at the surface. Connecting the motor to the variable speed drive is a cable Because the motor can be a very long distance from the variable speed drive, the voltage drop in the cable is very significant.
Wellbore characteristics and production goals determine the design of the ESP system, including the choice of motor and target operating speed, often described in terms of revolutions per minute (RPM). Because of different viscosities, densities, well flowing characteristics, and the like, it is desirable to vary to the speed of the motor. Typically, the output voltage of the variable speed drive is set to produce rated voltage at the motor terminals when the motor is operating at or near ratings. The output voltage of the variable speed drive is then varied linearly with the operating frequency to maintain a constant V/Hz ratio at the terminals of the variable speed drive.
Varying the output voltage of the variable speed drive linearly with operating frequency has a known problem. For example, if the operating speed of the motor is decreased for any reason, the load on the motor decreases because the pump load is a function of the revolutions per minute. A decreased load on the motor results in a lower motor current, which in turn decreases the voltage drop in the cable. Because the motor can be a very long distance from the variable speed drive, a decreased voltage drop in the cable is very significant. The end result is that the motor terminal voltage is too high, which can cause extra heating of the motor, decreased efficiency, and saturation of the core leading to other problems, as understood by those skilled in the art.
Whereas varying the output voltage of the variable speed drive with the operating frequency to maintain a constant V/Hz ratio at the terminals of the variable speed drive produces a linear V/HZ curve, one alternative approach known in the industry to mitigate this problem is to employ a “shaped” V/Hz curve. Rather than linear curve, the “shaped” V/Hz curve may be a piecewise continuous curve, or similar curve as understood by those skilled in the art, based on the expected motor load for a centrifugal pump.